The present invention relates to a valve operating apparatus of the DOHC engine used for automobile or the like.
In the valve operating mechanism for opening and closing the intake/exhaust valves of an engine, a DOHC direct acting method is known, wherein two cam shafts for driving respectively the intake valve and the exhaust valve are disposed on a cylinder head and tappets on the head of the intake/exhaust valves are directly pressed by a cam to open/close the intake/exhaust valves.
As for the valve operating mechanism of DOHC method, in a valve operating apparatus proposed in the Japanese Patent Publication No. HEI 5-17362, among two cam shafts on a cylinder head, at one face side of an engine, the motive power from a crank shaft is transmitted to one end of a first cam shaft by means of belt or the like and, at the same time, at the other face side,the other end of this first cam shaft and one end of a second cam shaft are interlocked and linked by gears, to transmit the power from the first cam shaft to the second cam shaft.
On the other hand, a rotational driving force applied from the crank shaft acts on these cam shafts, in opposition to the reaction from valve spring or others of the intake/exhaust valves. As a consequence, the cam shaft should be a shaft diameter thickened to provide a rigidity sufficient for preventing deformation such as torsion or flexion or the like under said rotational driving force.
Now, in the interlocking type valve operating apparatus proposed in said Publication, as a stronger rotational driving force acts on the first cam shaft to which the power of the crank shaft is transmitted directly, the first cam shaft should be formed with a shaft diameter having sufficient thickness to resist such a strong rotational driving force.
In the interlocking type valve operating apparatus of said Publication, the diameters of the first cam shaft and the second cam shaft being identical, the shaft diameter of the second shaft and bearings supporting the same would become unnecessary dimension.
As a result, friction or rotation inertia moment of the whole cam shaft system increases, energy consumption increases in rotational transmission system, moreover, when this rotational transmission is ensured by timing belt or timing chain, the tension of these transmission means increases as much and reduces their durability, and additionally, when it is assured by a timing gear, the backlash will increase the teeth knocking noise.
On the other hand, in high performance small stroke volume diesel engines, intake mixture flow is increased commonly by reducing the cylinder bore diameter and, at the same time, increasing the cam lift. However, as the cam lift increases, the protrusion of the cam portion increases as much. As the consequence, when the DOHC direct acting method wherein a cam shaft is placed side by side with the cam driving method is adopted, the cam portion of one cam shaft and the cam portion, or the stem portion of the other cam shaft may interfere easily, so their arrangement should avoid such interference.
However, if the interval between cam shafts is increased keeping the inclination of the valve shaft of the intake/exhaust valves in order to prevent such cam interference, the distance between valve disks will also increase. Moreover, as the diameter of the cylinder bore containing the intake/exhaust valves is limited by the engine volume or length, if the distance between valve disks is increased without changing the intake/exhaust valves diameters, the cylinder bore can no more contain the intake/exhaust valves.
As a consequence, to avoid interference of cam portions, the distance between cam shaft centers is increased without increasing the interval between valve portions of the intake/exhaust valves, by increasing the inclination of valve shafts of the intake/exhaust valves and arranging valve shafts in V-form, in place of increasing the distance between valve disks.
However, when the intake/exhaust valves are inclined largely by increasing the inclination of the valve shafts as in the case of this V-form arrangement, the valve portions of the intake/exhaust valves will be disposed inclined on the upper part of the cylinder bore, and the central portions of the arrangement of respective valves form a space convex upward, in a way to increase spaces called dead volume which can not contribute effectively to the combustion. The increase of this dead volume deteriorates the combustion to generate HC or smoke, resulting in the lowering of the engine performance. This will cause a particularly serious problem in case of higher compression ratio diesel engines.